Ch.6 Puberty

the onset of puberty involves the capability of the hypothalamic neurons to produce high frequency, low amplitude GnRH pulses, which can be influenced by energy metabolism/body size and appropriate exposure to external modulators.

testosterone’s affect on the hypothalamus

during embryogenesis, testosterone crosses the blood brain barrier and gets converted into estradiol. Estradiol defeminizes the hypothalamus and eliminates the GnRH surge center in the male

alpha-fetoprotein

a protein that binds to estradiol and prevents it from crossing the blood-brain barrier in the female

puberty

the ability to accomplish reproduction succesfully, which requires GnRH secretion at the appropriate frequency and quanities.

criteria that can define the onset of puberty in females 

• age at first estrus (becomes sexually receptive)

• age at first ovulation (difficult to determine) 

• age at which a female can support pregnancy without deleterious effects (most applicable) 

criteria that can define the onset of puberty in males

• age when behavioral traits are expressed

• age at first ejaculation

• age when spermatozoa first appear in the ejaculate

• age when the ejaculate contains a threshold of number of spermatozoa

2 factors that impact the development of the hypothalamic GnRH neurons in females

• development of a threshold body size and/or composition

• exposure to certain environmental or social cues

external factors that modulate the timing of puberty in females

• season during which the animla was born (sheep)

• photoperiod that the animal is experiencing during the onset of puberty (sheep)

• the presence of absence of the opposite sex during the peripubertal period (swine and cattle)

• density of the groups (within the same sex) in which the animals are housed (swine)

major factor limiting the onset of puberty

the failure of the hypothalamus to secrete sufficient quanitites of GnRH to cause gonadotropin release.

the anterior pituitary and gonads are known to be able to produce their hormones if stimulated by exogenous GnRH

tonic GnRH center responsibility

regulates the pulse frequency of GnRH in both males and females

surge center responsibility

GnRH neurons fire frequently and release large quantities of GnRH in order to cause the preovulatory LH surge, resulting in ovulation in females only

prepubertal surge center

is still sensitive to estradiol, but there is insufficient amounts of circulating estradiol to trigger the surge center

feedback loop in between the GnRH neurons and gonad hormones in females 

positive (surge center) and negative (tonic center)

sensitivity of tonic center to testosterone/estradiol in males during onset of puberty

the tonic center becomes less and less sensitive to the negative feedback of testosterone and estradiol, allowing the hypothalamus to secrete more and more GnRH. 

how does the body know when to place a priority on reproduction

hypothalamic neurons that regulate GnRH secretion detects "moment-to-moment” changes in blood glucose and fatty acids

leptin

a hormonal peptide that is secreted by adipocytes, whose concentration is directly related to the amount of fat in the body.

leptin target tissues

receptors in the liver, kidney, heart, skeletal muscles, pancreas, anterior pituitary, and hypothalamus

potential role of leptin

mediating the onset of puberty in mammals and signaling key hypothalamic GnRH secreting neurons that nurtitional status is adequate

kisspeptin neurons location, stimulus, and response

located in the hypothalamus; may be stimulated by leptin, fatty acids, and glucose; and stimulate the firing of GnRH neurons

affect on a gilt being housed in small groups of other gilts on puberty

gilts house in small groups have delayed puberty compared to gilts housed in larger groups 

affect of the presence of a male on on the onset of puberty in gilts

being exposed to a boar (visually or physically) will hasten the onset of puberty than gilts not exposed to a boar